Electroplating method

ABSTRACT

A substrate with a through-hole is immersed in a plating solution in a plating tank. A pair of anodes are disposed in the plating solution in the plating tank in facing relation to face and reverse sides, respectively, of the substrate in the plating solution. A plurality of plating processes are performed on the face and reverse sides by supplying pulsed currents respectively between the face side of the substrate and one of the anodes which faces the face side of the substrate, and between the reverse side of the substrate and the other anode which faces the reverse side of the substrate. A reverse electrolyzing process is performed on the face and reverse sides between adjacent plating processes by supplying currents in an opposite direction to the pulsed currents respectively between the face side of the substrate and one of the anodes, and between the reverse side of the substrate and the other anode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroplating method forsimultaneously plating both the face and reverse sides of a substratewhich has a through-hole vertically penetrating in its interior to filla plated film of metal such as copper or the like into the through-hole.

2. Description of the Related Art

A technique of forming a plurality of through-vias of a metal,vertically penetrating through a substrate, is known as a method toelectrically connect the layers of a multi-layer stack of substratessuch as semiconductor substrates. It is customary to make verticalthrough-vias in a substrate by simultaneously plating both the face andreverse sides of a substrate, which has through-holes verticallypenetrating in its interior, thereby to fill a plated film of metal intothe through-holes.

There is known an electroplating apparatus for producing through-vias(see Japanese Patent No. 4138542). This electroplating apparatusincludes a substrate holder for holding a substrate while exposingcertain areas on its face and reverse sides and sealing peripheral areasaround the certain areas, and a pair of anodes disposed in facingrelation to the face and reverse sides, respectively, of the substratethat is held by the substrate holder. The substrate held by thesubstrate holder and the anodes are immersed in a plating solution, andthen voltages are applied between the substrate and the anodes tosimultaneously plate the face and reverse sides of the substrate, whichhas vertical through-holes defined therein, embedding metal such ascopper in the through-holes.

FIGS. 1A through 1D are diagrams illustrating, in a sequence of processsteps, a process for filling a plated film into a through-hole definedin a substrate to form a through-via therein (see Japanese Patent No.4248353).

As shown in FIG. 1A, a substrate W is prepared which includes a base 100with a vertical through-hole 100 a defined therein, and a barrier layer102 made of Ti or the like and a seed layer 104, as an electric feedlayer, which cover all the surfaces of the base 100 including innersurfaces of the through-hole 100 a. The face and reverse sides of thesubstrate W are simultaneously plated to deposit a plated film 106 ofmetal such as copper or the like on the face and reverse sides of thesubstrate W and in the through-hole 100 a, as shown in FIG. 1B. Theplated film 106 in the through-hole 100 a has its maximum thickness atits center along the in-depth direction thereof. Then, as shown in FIG.1C, the plated film 106 is grown until the tip ends of layers of theplated film 106 that have grown from the wall surfaces of thethrough-hole 100 a are joined to each other at the center of thethrough-hole 100 a along the in-depth direction thereof. The center ofthe through-hole 100 a along the in-depth direction thereof is thusblocked by the plated film 106, forming recesses 108 above and below theclosed region. The plating process is further continued to grow theplated film 106 in the recesses 108 until the recesses 108 are filled upwith the plated film 106, as shown in FIG. 1D. In this manner, athrough-via made up of the plated film 106 is produced in the substrateW.

There has been proposed an electroplating method for fillingthrough-holes defined in a substrate with a plated film of metal (seeJapanese Patent Laid-Open Publication No. 2008-513985). According tothis electroplating method, a forward pulsed current is supplied to flowbetween a substrate as a cathode and an anode, and a reverse pulsedcurrent, which flows in an opposite direction to the forward pulsedcurrent, is also is supplied to flow between the substrate and theanode, for thereby fully or substantially fully filling the center ofthe through-hole.

There has also been proposed a method to prevent whiskers from beinggenerated in plating a printed wiring substrate or the like with copper(see Japanese Patent Laid-Open Publication No. 2010-95775). According tothis method, a DC power source for applying a DC voltage between acathode and an anode has its polarity reversible. The printed wiringsubstrate is electroplated alternately under a normal DC voltage and areversed DC voltage, i.e., alternately in a normal electrolyzing cyclein which the printed wiring substrate serves as a cathode and a reverseelectrolyzing cycle in which the printed wiring substrate serves as ananode.

SUMMARY OF THE INVENTION

In order to form a through-via in the form of a plated film free ofdefects such as voids or the like therein in a substrate, as shown inFIGS. 1A through 1D, it is ideal that the plated film be grownpreferentially at the center of the through-hole along the in-depthdirection thereof until the center of the through-hole 100 a is blockedby the plated film 106, and then the plating process be furthercontinued. However, it is generally practically difficult to attempt tomeet the ideal requirements and at the same time to fill the plated filmefficiently into the through-hole to shorten the time required toperform the plating process. Stated otherwise, the conventionalelectroplating processes have failed to achieve both the ideal fillingof the plated film into the through-hole and the efficient filling ofthe plated film into the through-hole with a higher average platingcurrent during plating.

The present invention has been made in view of the above situation. Itis therefore an object of the present invention to provide anelectroplating method for efficiently filling a plated film into athrough-hole with a higher average plating current during plating toshorten the time required to perform the plating process and alsoideally filling the plated film into the through-hole.

In order to achieve the above object, the present invention provides anelectroplating method comprising: immersing a substrate with athrough-hole defined therein in a plating solution in a plating tank;disposing a pair of anodes in the plating solution in the plating tankin facing relation to face and reverse sides, respectively, of thesubstrate in the plating solution; performing a plurality of platingprocesses, each for a predetermined period, on the face and reversesides of the substrate by supplying pulsed currents respectively betweenthe face side of the substrate and one of the anodes which faces theface side of the substrate, and between the reverse side of thesubstrate and the other of the anodes which faces the reverse side ofthe substrate; and performing a reverse electrolyzing process on theface and reverse sides of the substrate between adjacent ones of theplating processes by supplying currents in an opposite direction to thepulsed currents in the plating processes respectively between the faceside of the substrate and one of the anodes which faces the face side ofthe substrate, and between the reverse side of the substrate and theother of the anodes which faces the reverse side of the substrate.

Since the plural plating processes are performed, each for apredetermined period, on the face and reverse sides of the substrate bysupplying pulsed currents respectively between the face side of thesubstrate and one of the anodes which faces the face side of thesubstrate, and between the reverse side of the substrate and the otherof the anodes which faces the reverse side of the substrate, it ispossible to fill a plated film into the through-hole efficiently with anincreased average current value for thereby shortening a period of timerequired to plate the substrate. The reverse electrolyzing processperformed between the plating processes is effective to dissolve platedfilms deposited on corners of the through-hole. Therefore, it ispossible to ideally fill the plated film into the through-hole bygrowing the plated film preferentially at the center of the through-holealong the in-depth direction thereof.

In a preferred aspect of the present invention, each of the pulsedcurrents comprises a PR pulsed current represented by an alternaterepetition of a current flowing in a forward direction and a currentflowing in a reverse direction.

The reverse electrolyzing process is repeatedly performed between theplating processes using the PR pulsed currents, thereby preventing fineirregularities from being produced by an abnormal deposition onmicroscopic surfaces of the plated film and hence preventing fine voidsfrom being formed in the plated film due to such fine irregularities.

In a preferred aspect of the present invention, each of the pulsedcurrents comprises an on/off pulsed current represented by an alternaterepetition of the supply and non-supply of a plating current which flowsin a forward direction.

Since the on/off pulsed current provides non-plating periods forsupplying no plating current in the plating process, the metal ionconcentration in the plating solution within the through-hole isrecovered in the non-plating period for thereby preventing defects suchas voids or the like from being formed in the plated film.

In a preferred aspect of the present invention, each of the pulsedcurrents comprises a composite pulsed current represented by acombination of two pulsed currents having different current values.

Since the plated film is continuously grown in the plating process withthe composite pulsed current, the plated film is prevented from beingdissolved into the plating solution in the plating process.

In a preferred aspect of the present invention, the plating processestogether with the reverse electrolyzing process are performed togradually increase an average current density as the substrate isprogressively plated.

As the through-hole is gradually filled with the plated film in theplating process, the substantive aspect ratio of the through-holechanges. When the substantive aspect ratio of the through-hole changes,it is possible to efficiently fill the plated film into the through-holein a manner to match the changing substantive aspect ratio by increasingthe average current density in the plating process. Consequently, theperiod of time required to plate the substrate can be further shortened.

In a preferred aspect of the present invention, the reverseelectrolyzing process is performed a plurality of times before and aftera normal electrolyzing cycle in which a pulsed current is supplied inthe forward direction.

The reverse electrolyzing process is performed with a negative cathodecurrent density in the range from −30 to −40 ASD at a pulse pitch in therange from 0.1 to 10 ms, for example. Depending on the aspect ratio of athrough-hole defined in the substrate, it may not be possible to ideallyfill a plated film preferentially at the center of the through-hole inthe in-depth direction thereof according to a reverse electrolyzingprocess at a pulse pitch that is shorter than 1.0 ms. However, if thereverse electrolyzing process is repeatedly performed a plurality oftimes at a pulse pitch shorter than 1.0 ms before and after a normalelectrolyzing cycle in which a pulsed current is supplied in the forwarddirection, then it is possible to ideally fill a plated film into such athrough-hole.

According to the present invention, as described above, the pluralplating processes are performed, each for a predetermined period, on theface and reverse sides of the substrate by supplying pulsed currentsrespectively between the face side of the substrate and one of theanodes which faces the face side of the substrate, and between thereverse side of the substrate and the other of the anodes which facesthe reverse side of the substrate. Accordingly, it is possible to fill aplated film into the through-hole efficiently with an increased averagecurrent value for thereby shortening a period of time required to platethe substrate. The reverse electrolyzing process performed between theplating processes is effective to dissolve plated films deposited oncorners of the through-hole. Therefore, it is possible to ideally fillthe plated film into the through-hole by growing the plated filmpreferentially at the center of the through-hole along the in-depthdirection thereof.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D are diagrams illustrating, in a sequence of processsteps, a process for filling a plated film into a through-hole definedin a substrate to form a through-via therein;

FIG. 2 is a vertical sectional front view schematically showing anelectroplating apparatus which is used to carry out an electroplatingmethod according to the present invention;

FIG. 3 is a front view of a substrate holder of the electroplatingapparatus shown in FIG. 2;

FIG. 4 is a plan view of the substrate holder of the electroplatingapparatus shown in FIG. 2;

FIG. 5 is a bottom view of the substrate holder of the electroplatingapparatus shown in FIG. 2;

FIG. 6 is a cross-sectional view taken along line K-K of FIG. 3;

FIG. 7 is a view of the substrate holder as viewed along the arrow A inFIG. 6;

FIG. 8 is a view of the substrate holder as viewed along the arrow B inFIG. 6;

FIG. 9 is a view of the substrate holder as viewed along the arrow C inFIG. 6;

FIG. 10 is a cross-sectional view taken along line D-D of FIG. 7;

FIG. 11 is a cross-sectional view taken along line E-E of FIG. 7;

FIG. 12 is a cross-sectional view taken along line F-F of FIG. 3;

FIG. 13 is a cross-sectional view taken along line G-G of FIG. 7;

FIG. 14 is a cross-sectional view taken along line H-H of FIG. 8;

FIG. 15 is a front view of an anode holder, which is holding aninsoluble anode therein, of the electroplating apparatus shown in FIG.2;

FIG. 16 is a cross-sectional view of the anode holder, which is holdingthe insoluble anode therein, of the electroplating apparatus shown inFIG. 2;

FIG. 17 is an enlarged cross-sectional view of the main portion ofanother substrate holder;

FIG. 18 is an enlarged cross-sectional view of the main portion of thesubstrate holder shown in FIG. 17;

FIG. 19 is an enlarged cross-sectional view of the main portion of thesubstrate holder shown in FIG. 17;

FIG. 20 is a graph showing the relationship between the cathode currentdensity and time for an example of a plating current that is suppliedbetween a substrate surface and an anode;

FIG. 21 is an enlarged fragmentary cross-sectional view showing themanner in which a plated film is grown preferentially at the center of athrough-hole along the in-depth direction thereof when a reverseelectrolyzing process is performed after a plating process;

FIG. 22 is an enlarged fragmentary cross-sectional view schematicallyshowing the manner in which fine irregularities are produced by anabnormal deposition on microscopic surfaces of the plated film in theplating process;

FIG. 23 is a graph showing the relationship between the cathode currentdensity and time for another example of a plating current that issupplied between a substrate surface and an anode;

FIGS. 24A and 24B are enlarged fragmentary cross-sectional viewsschematically showing the manner in which a plated film embedded in athrough-hole is excessively dissolved into the plating solution untilfinally voids are formed in the plated film;

FIG. 25 is a graph showing the relationship between the cathode currentdensity and time for still another example of a plating current that issupplied between a substrate surface and an anode;

FIG. 26 is a graph showing the relationship between the cathode currentdensity and time for yet another example of a plating current that issupplied between a substrate surface and an anode;

FIG. 27 is a graph showing the relationship between the cathode currentdensity and time for yet still another example of a plating current thatis supplied between a substrate surface and an anode;

FIG. 28 is a graph showing the relationship between the cathode currentdensity and time for a further example of a plating current that issupplied between a substrate surface and an anode;

FIG. 29 is a graph showing the relationship between the cathode currentdensity and time for a still further example of a plating current thatis supplied between a substrate surface and an anode;

FIG. 30 is a graph showing the relationship between the cathode currentdensity and time for a yet further example of a plating current that issupplied between a substrate surface and an anode;

FIG. 31 is a graph showing the relationship between the cathode currentdensity and time for a yet still further example of a plating currentthat is supplied between a substrate surface and an anode; and

FIG. 32 is a graph showing the relationship between the cathode currentdensity and time for another example of a plating current that issupplied between a substrate surface and an anode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings. FIG. 2 is a vertical sectional frontview schematically showing an electroplating apparatus 50 which is usedto carry out an electroplating method according to the presentinvention. As shown in FIG. 2, the electroplating apparatus 50 includesa plating tank 51 holding a plating solution Q therein, and a substrateholder 10 holding a substrate W such as a semiconductor wafer or thelike and suspended vertically in the plating solution Q. The platingsolution Q with the substrate holder 10 immersed therein has a surfacelevel L at the upper end of the plating tank 51, as shown in FIG. 2. Twoinsoluble anodes 52 supported on respective anode holders 58 aredisposed in the plating tank 51 in facing relation to respectiveopposite surfaces, i.e., face and reverse sides, of the substrate W heldby the substrate holder 10. As shown in FIG. 3, the substrate holder 10includes a first holding member 11 having a circular hole 11 a definedtherein and a second holding member 12 having a circular hole 12 adefined therein. The first holding member 11 and the second holdingmember 12 serve to hold the substrate W therebetween. The insolubleanodes 52 are circular in shape and substantially identical in size tothe circular holes 11 a, 12 a in the first and second holding members11, 12.

Two regulation plates 60 made of an insulating material are disposedbetween the substrate holder 10 and the respective insoluble anodes 52in the plating tank 51. The regulation plates 60 have respectivecircular holes defined therein which are similar in shape to thecircular holes 11 a, 12 a in the first and second holding members 11,12. The insoluble anodes 52 are electrically connected to respectivewires 61 a extending from respective terminals of plating power sources53 each capable of changing the direction in which a current suppliedthereby and also changing the value of the current. The plating powersources 53 have other terminals electrically connected to respectivewires 61 b which are connected respectively to terminal plates 27, 28(see FIG. 3) of the substrate holder 10. The plating power sources 53are also electrically connected to a controller 59 which individuallycontrols the plating power sources 53.

Two stirring paddles 62 are disposed between the substrate W held by thesubstrate holder 10 and the respective regulation plates 60 in theplating tank 51. The stirring paddles 62 are movable back and forthparallel to the substrate W held by the substrate holder 10 for stirringthe plating solution Q. The electroplating apparatus 50 also includes anouter tank 57 disposed around the plating tank 51 for holding theplating solution Q which has overflowed the plating tank 51. The platingsolution Q, which has overflowed the plating tank 51 into the outer tank57, is circulated through a constant-temperature unit 55 and a filter 56back into the plating tank 51 from its bottom by a plating solutioncirculation pump 54.

FIG. 3 is a front view of a substrate holder 10. FIG. 4 is a plan viewof the substrate holder 10. FIG. 5 is a bottom view of the substrateholder 10. FIG. 6 is a cross-sectional view taken along line K-K of FIG.3. FIG. 7 is a view of the substrate holder 10 as viewed along the arrowA in FIG. 6. FIG. 8 is a view of the substrate holder 10 as viewed alongthe arrow B in FIG. 6. FIG. 9 is a view of the substrate holder 10 asviewed along the arrow C in FIG. 6. FIG. 10 is a cross-sectional viewtaken along line D-D of FIG. 7. FIG. 11 is a cross-sectional view takenalong line E-E of FIG. 7. FIG. 12 is a cross-sectional view taken alongline F-F of FIG. 3. FIG. 13 is a cross-sectional view taken along lineG-G of FIG. 7. FIG. 14 is a cross-sectional view taken along line H-H ofFIG. 8.

As shown in FIG. 3, the first holding member 11 and the second holdingmember 12, each of a planar shape, of the substrate holder 10 haverespective lower ends pivotally coupled to each other by a hingemechanism 13. The hinge mechanism 13 has two hooks 13-1 of syntheticresin, e.g., HTPVC, which are fixed to the second holding member 12. Thehooks 13-1 are angularly movable supported on a lower end of the firstholding member 11 by a hook pin 13-2 made of stainless steel, e.g., SUS303. The first holding member 11 is made of synthetic resin, e.g.,HTPVC, and has a substantially pentagonal shape. The circular hole 11 ais centrally defined in the first holding member 11, as shown in FIG. 7.As shown in FIG. 3, a T-shaped hanger 14 made of synthetic resin, e.g.,HTPVC, is integrally formed with an upper end of the first holdingmember 11. The second holding member 12 is made of synthetic resin,e.g., HTPVC, and has a substantially pentagonal shape. The circular hole12 a is centrally defined in the second holding member 12.

When the first holding member 11 and the second holding member 12 areturned about the hinge mechanism 13 into superposed relation to eachother, i.e., when the substrate holder 10 is closed, the first holdingmember 11 and the second holding member 12 are held together by left andright clamps 15, 16. The left and right clamps 15, 16, each made ofsynthetic resin, e.g., HTPVC, have respective groove 15 a, 16 a forreceiving therein the side marginal edges of the first holding member 11and the second holding member 12 that are superposed one on the other.The left and right clamps 15, 16 have lower ends angularly movablysupported on the lower ends of the opposite sides of the first holdingmember 11 by respective pins 17, 18.

As shown in FIG. 7, a seal ring 19 is mounted on a surface of the firstholding member 11 which faces the second holding member 12, and extendsaround the hole 11 a. As shown in FIG. 9, a seal ring 20 is mounted on asurface of the second holding member 12 which faces the first holdingmember 11, and extends around the hole 12 a. The seal rings 19, 20 aremade of rubber, e.g., silicone rubber. An O-ring 29 is mounted on thesurface of the second holding member 12 which faces the first holdingmember 11, and extends around the seal ring 20.

The seal rings 19, 20, each of a rectangular cross-sectional shape, haverespective ridges 19 a, 20 a projecting radially inwardly from andextending along inner circumferential edges thereof. When the firstholding member 11 and the second holding member 12 are superposed one onthe other with the substrate W interposed therebetween, the ridges 19 a,20 a press the respective surfaces of the substrate W and are held inclose contact therewith, defining a watertight space free of the platingsolution Q between the O-ring 29 and the ridges 19 a, 20 a that arepositioned radially outwardly of the holes 11 a, 12 a. As shown in FIGS.7 and 10, eight substrate guide pins 21 for positioning the substrate Ware mounted on the surface of the first holding member 11 which facesthe second holding member 12, radially outwardly of the hole 11 a, andproject through the seal ring 19.

As shown in FIGS. 7, 11 and 12, six conductive plates 22 are mounted onthe surface of the first holding member 11 which faces the secondholding member 12 around the hole 11 a. As shown in FIG. 11, three outof the six conductive plates 22 are held in electric contact with theseed layer 104 (see FIGS. 1A through 1D) on one of the surfaces, e.g.,the face side, of the substrate W through conductive pins 23. As shownin FIG. 12, the other three conductive plates 22 held in electriccontact with the seed layer 104 on the other surface, e.g., the reverseside, of the substrate W through conductive pins 23.

The three conductive plates 22 which are held in electric contact withthe seed layer 104 on one of the surfaces, e.g., the face side, of thesubstrate W are electrically connected to respective electrode terminals27 a, 27 b, 27 c (see FIG. 4) provided on the terminal plate 27 of thehanger 14 through insulative covered wires 26 extending through a wireslot 25 (see FIG. 13). The other three conductive plates 22 which areheld in electric contact with the seed layer 104 on the other surface,e.g., the reverse side, of the substrate W are electrically connected torespective electrode terminals 28 a, 28 b, 28 c (see FIG. 4) provided onthe other terminal plate 28 of the hanger 14 through insulative coveredwires 26 extending through a wire slot 25 (see FIG. 13). As shown inFIGS. 7 and 13, the insulative covered wires 26 are held in position bywire holders 30 made of a synthetic resin, e.g., PVC.

The substrate holder 10 operates as follows: When the first holdingmember 11 and the second holding member 12 are turned about the hingemechanism 13 away from each other, i.e., when the substrate holder 10 isopen, the substrate W is placed in an area on the first holding member11 which is surrounded by the eight substrate guide pins 21. Thesubstrate W is now positioned in place on the first holding member 11.The first holding member 11 and the second holding member 12 are turnedabout the hinge mechanism 13 toward each other, i.e., the substrateholder 10 is closed. The left and right clamps 15, 16 are then angularlymoved about the pins 17, 18 until the side marginal edges of the firstholding member 11 and the second holding member 12 are inserted in therespective grooves 15 a, 16 a of the left and right clamps 15, 16. Thesubstrate W, which is positioned in place on the first holding member11, is now held between the first holding member 11 and the secondholding member 12.

The O-ring 29 and the ridges 19 a, 20 a of the seal rings 19, 20 jointlydefine a watertight space free of the plating solution Q therebetween.At this time, the outer circumferential edge area of the substrate W,which is positioned radially outwardly of the ridges 19 a, 20 a, ispositioned in the watertight space, and the surface areas of theopposite surfaces of the substrate W, which are coextensive with theholes 11 a, 12 a of the first holding member 11 and the second holdingmember 12, are exposed to the holes 11 a, 12 a. The three of the sixconductive plates 22, which are held in electric contact with the seedlayer 104 on one of the surfaces of the substrate W, are electricallyconnected to the electrode terminals 27 a, 27 b, 27 c provided on theterminal plate 27 of the hanger 14, and the other three conductiveplates 22, which are held in electric contact with the seed layer 104 onthe other surface of the substrate W, are electrically connected to theelectrode terminals 28 a, 28 b, 28 c provided on the terminal plate 28of the hanger 14.

FIG. 15 is a front view of the anode holder 58, which is holding theinsoluble anode 52 therein, of the electroplating apparatus shown inFIG. 2, and FIG. 16 is a cross-sectional view of FIG. 15. In thisembodiment, in order to prevent anodes from being dissolved by anadditive(s) of the plating solution, the insoluble anodes 52, each ofwhich comprises an anode body of titanium coated with iridium oxide, forexample, are used.

As shown in FIGS. 15 and 16, each of the anode holders 58 includes aholder body 70 having a central hole 70 a defined therein, a closureplate 72 disposed on a reverse side of the holder body 70 and closingthe central hole 70 a, a circular support plate 74 disposed in thecentral hole 70 a of the holder body 70 and holding the insoluble anode52 on its surface such that the insoluble anode 52 is positioned in thecentral hole 70 a, and an annular anode mask 76 mounted on a face sideof the holder body 70 in surrounding relation to the central hole 70 a.The support plate 74 has a channel 74 a defined therein which housestherein a conductive plate 78 which is electrically connected to thewire 61 a extending from the plating power source 53. The conductiveplate 78 extends to a central area of the support plate 74 where theconductive plate 78 is electrically connected to the insoluble anode 52.

A separating membrane 80 in the form of a neutral membrane is disposedin covering relation to the surface of the insoluble anode 52 that ispositioned in the central hole 70 a of the holder body 70. Theseparating membrane 80 has its peripheral edge gripped in position bythe holder body 70 and the anode mask 76, and is fastened to the holderbody 70. The anode mask 76 is fastened to the holder body 70 by screws82, and the closure plate 72 is also fastened to the holder body 70 byscrews.

When the anode holder 58 is immersed in the plating solution Q, theplating solution Q enters a gap between the insoluble anode 52 and thesupport plate 74 in the central hole 70 a of the holder body 70.

The insoluble anode 52 and the separating membrane 80 are used for thefollowing reasons: An additive to be added to the plating solution Qincludes a component for promoting the formation of monovalent copper,which impairs the function of other additives because it causesoxidative decomposition of the other additives. As a result, solubleanodes cannot be used. When insoluble anodes are used, the insolubleanodes produce an oxygen gas in the vicinity thereof, and part of theproduced oxygen gas is dissolved into the plating solution Q, increasingthe concentration of dissolved oxygen. The increased concentration ofdissolved oxygen tends to cause oxidative decomposition of theadditives. Therefore, the separating membrane 80 in the form of aneutral membrane is desirably disposed in covering relation to thesurface of the insoluble anode 52 to prevent the components of theadditives near the substrate W from being adversely affected even ifthey are subject to oxidative decomposition in the vicinity of theinsoluble anode 52.

It is also desirable to bubble or aerate the plating solution Q in thevicinity of the insoluble anode 52 with air or nitrogen supplied via,e.g., an aeration tube, not shown, for preventing the concentration ofdissolved oxygen from being unduly rising on the insoluble anode 52side.

Since the surface of the insoluble anode 52 held by the anode holder 58is covered with the separating membrane 80 and the insoluble anode 52 isdisposed to allow the separating membrane 80 to face the substrate Wthat is held by the substrate holder 10 and disposed in the plating tank51, it is possible to prevent an oxygen gas from being produced in thevicinity of the insoluble anode 52 and dissolving into the platingsolution when the plating solution Q is bubbled or aerated and hence toprevent the concentration of dissolved oxygen in the plating solution Qfrom increasing.

The electroplating apparatus 50 thus constructed operates as follows:The substrate holder 10, which is holding the substrate W whose face andreverse sides are exposed, is placed in the plating solution Q in theplating tank 51 such that one of the surfaces of the substrate W, e.g.,the face side thereof, faces one of the insoluble anodes 52 and theother surface of the substrate W, e.g., the reverse side thereof, facesthe other insoluble anode 52. The plating power sources 53 supplyplating currents that are controlled by the controller 59 respectivelybetween the face side of the substrate W and the insoluble anode 52which faces the face side of the substrate W, and between the reverseside of the substrate W and the insoluble anode 52 which faces thereverse side of the substrate W, thereby simultaneously plating the faceand reverse sides of the substrate W. If necessary, when the face andreverse sides of the substrate W are plated, the stirring paddles 62 aremoved back and forth parallel to the substrate W to stir the platingsolution Q. In this manner, as shown in FIGS. 1A through 1D, a platedfilm 106 is grown in the through-hole 100 a defined in the substrate W.

FIGS. 17 through 19 show enlarged cross-sectional views of anothersubstrate holder taken in different cross-sectional planes,respectively. The substrate holder shown in FIGS. 17 through 19 isdifferent from the above-described substrate holder as follows: As shownin FIG. 17, the substrate holder includes elastic conductive plates 90,92 having respective proximal ends fastened to the first holding member11 and the second holding member 12, instead of the conductive pins 22,23 shown in FIGS. 11 and 12. When the substrate W is held by the firstholding member 11 and the second holding member 12, distal free ends ofthe elastic conductive plates 90, 92 are elastically held against theface and reverse sides, respectively, of the substrate W in electriccontact with the seed layers 104 (see FIGS. 1A through 1D) on the faceand reverse sides of the substrate W.

As shown in FIGS. 18 and 19, the substrate holder also includes sealring holders 94, 96 for holding the seal rings 19, 20, respectively. Theseal ring holders 94, 96 are fastened to the first holding member 11 andthe second holding member 12, respectively. The seal ring holders 94, 96have respective arrays of alternate guide teeth 97, 98 for positioningthe substrate W, instead of the substrate guide pins 21 shown in FIGS. 7and 10. The guide teeth 97, 98 are disposed at respective positionsalong the circumferential direction of the seal ring holders 94, 96. Theguide teeth 97, 98 have respective tapered surfaces 97 a, 98 a on innerperipheral surfaces thereof near free ends thereof. When the substrate Wis held by the first holding member 11 and the second holding member 12,the outer circumferential edge of the substrate W is held in contactwith and guided by the tapered surfaces 97 a, 98 a to position thesubstrate W.

FIG. 20 shows the relationship between the cathode current density andtime for an example of a plating current that is supplied between asurface of the substrate W and the insoluble anode 52 disposed in facingrelation to the surface of the substrate W. The plating current, whichis supplied between the reverse side of the substrate W and theinsoluble anode 52 which faces the reverse side of the substrate W, isheld in synchronism with the plating current which is supplied betweenthe face side of the substrate W and the insoluble anode 52 which facesthe face side of the substrate W. However, these plating currents do notneed to be synchronized with each other, and hence the present inventionshould not be limited by whether the above plating currents are to besynchronized with each other or not. The relationship between thecathode current density and time will be described with reference toFIG. 20 for a plating current that is supplied between a surface of thesubstrate W and the insoluble anode 52 disposed in facing relationthereto.

In the example shown in FIG. 20, a plating process A in which a pulsedcurrent is supplied between the surface of the substrate W and theinsoluble anode 52 for plating the surface of the substrate W for apredetermined period of time, and a reverse electrolyzing process B inwhich a current is supplied in a direction opposite to the currentsupplied in the plating process A between the surface of the substrate Wand the insoluble anode 52 are alternately repeated. The predeterminedperiod of time for which the plating process A is carried out is in therange from 50 to 100 ms, for example, and the predetermined period oftime for which the reverse electrolyzing process B is carried out is inthe range from 0.1 to 10 ms, or preferably from 0.5 to 1 ms, forexample.

As indicated by the imaginary lines in FIG. 20, a quiescent period C of0.05 ms, for example, in which no current is supplied between thesurface of the substrate W and the insoluble anode 52 may be insertedafter the reverse electrolyzing process B and before the plating processA. The quiescent period C can uniformize a metal ion distribution in theplating solution Q within the through-hole for efficiently filling theplated film into the through-hole. The quiescent period C may beinserted for its advantages in each of other examples to be describedbelow.

In the example shown in FIG. 20, the plating process A is carried out,using a PR pulsed current which is represented by an alternaterepetition of normal electrolyzing cycles at a pulse pitch P₁ in whichthe plating current flows in a forward direction, i.e., a platingdirection, with a positive cathode current density D₁ in the range from1 to 3 ASD (A/dm²), for example, and reverse electrolyzing cycles at apulse pitch P₂ in which the plating current flows in a reverse directionwith a negative cathode current density D₂ in the range from −0.05 to −4ASD, for example. The pulse pitch P₂ in the reverse electrolyzing cyclesof the PR pulsed current is of 0.5 ms, for example. The reverseelectrolyzing process B is carried out with a single pulse at a pulsepitch P₃ in the range from 0.1 to 10 ms, preferably from 0.5 to 1 ms,with a negative cathode current density D₃ in the range from −30 to −40ASD, for example.

Since the reverse electrolyzing process B with the negative cathodecurrent density D₃ in the range from −30 to −40 ASD, for example, iscarried out after the plating process A, as indicated by the imaginarylines in FIG. 21, a plated film 106 a, which tends to be deposited atthe corners of the through-hole 100 a, is dissolved into the platingsolution Q, thereby allowing the plated film 106 to grow preferentiallyat the center of the through-hole 100 a along the in-depth directionthereof, as indicated by the solid lines in FIG. 21.

As schematically shown in FIG. 22, fine irregularities 106 b are liableto be produced by an abnormal deposition on microscopic surfaces of theplated film 106 in the plating process. However, those fineirregularities 106 b are prevented from being produced by the reverseelectrolyzing cycles with the negative cathode current density D₂ in therange from −0.05 to −4 ASD, for example, according to the example shownin FIG. 20. The fine irregularities 106 b due to an abnormal depositionwould otherwise be joined to each other, forming fine voids in theplated film.

FIG. 23 shows the relationship between the cathode current density andtime for another example of a plating current that is supplied between asurface of the substrate W and the insoluble anode 52 disposed in facingrelation to the surface of the substrate W. The example shown in FIG. 23is different from the example shown in FIG. 20 in that a reverseelectrolyzing process B₁ is carried out by applying two pulses each at apulse pitch P₄ in the range from 0.1 to 10 ms, for example, preferablyfrom 0.5 to 1.0 ms before and after a normal electrolyzing cycle inwhich the plating current is applied in the forward direction.

The reverse electrolyzing process B with the negative cathode currentdensity D₃ in the range from −30 to −40 ASD, as shown in FIG. 20, iscarried out with the single pulse at the pulse pitch P₃ in the rangefrom 0.1 to 10 ms. If the pulse pitch P₃ is greater than 1 ms, then asschematically shown in FIG. 24A, the plated film 106 is excessivelydissolved into the plating solution, forming excessively dissolvedregions 112. As shown in FIG. 24B, the excessively dissolved regions 112have their open ends closed, tending to produce cat-eyed voids 114within the plated film 106 embedded in the through-hole 110 a.Therefore, the pulse pitch P₃ should preferably in the range from 0.1 to1.0 ms, and more preferably in the range from 0.5 to 1.0 ms.

However, depending on the aspect ratio of a through-hole defined in thesubstrate W, it may not be possible to perform an ideal embeddingprocess for ideally embedding a plated film preferentially at the centerof the through-hole along the in-depth direction thereof according to areverse electrolyzing process using a single pulse having a pulse pitchthat is shorter than 1.0 ms. The reverse electrolyzing process B₁ thatis carried out by applying two pulses, each at the pulse pitch P₄shorter than 1.0 ms, as shown in FIG. 23, makes it possible to ideallyfill a plated film into such a through-hole.

FIG. 25 shows the relationship between the cathode current density andtime for still another example of a plating current that is suppliedbetween a surface of the substrate W and the insoluble anode 52 disposedin facing relation to the surface of the substrate W. The example shownin FIG. 25 includes three different plating processes, i.e., a platingprocess (first plating process) A₁ in a first zone until the plated film106 in the through-hole 100 a is joined substantially at the centerthereof along the in-depth direction of the through-hole 100 a, as shownin FIGS. 1A through 1C, a plating process (second plating process) A₂ ina second zone for embedding the plated film 106 to a predeterminedthickness in the recesses 108 in the through-hole 100 a, as shown inFIGS. 1C and 1D, and a plating process (third plating process) A₃ in athird zone in which the danger of a pinch-off is reduced after the stageshown in FIG. 1D.

In FIG. 25, the first plating process A₁, the second plating process A₂and the third plating process A₃ are shown as being carried out onceeach before and after the reverse electrolyzing process B (see FIG. 20).However, each of the first plating process A₁, the second platingprocess A₂ and the third plating process A₃ is actually carried out anumber of times before and after the reverse electrolyzing process B.This also applies to each of other examples to be described below.

In the example shown in FIG. 25, each of the first plating process A₁,the second plating process A₂ and the third plating process A₃ iscarried out with an on/off pulsed current which is represented by analternate repetition of the supply and non-supply of a plating currentwhich flows in the forward direction, i.e., the plating direction, andhas a positive cathode current density D₁ in the range from 1 to 3 ASD,for example. The on/off pulsed current in the first plating process A₁has a pulse pitch P₅ shorter than the pulse pitch P₆ of the on/offpulsed current in the second plating process A₂ (P₅<P₆), and the pulsepitch P₆ of the on/off pulsed current in the second plating process A₂is shorter than the pulse pitch P₇ of the on/off pulsed current in thethird plating process A₃ (P₆<P₇). The on/off pulsed currents in thefirst, second and third plating processes A₁, A₂, A₃ have respectivedowntime pitches P₈, P₉, P₁₀ of the respective on/off pulsed currentsequal to each other (P₈=P₉=P₁₀). Therefore, the cathode current densityon average increases stepwise. Alternatively, the cathode currentdensity on average may increase gradually linearly.

Since the on/off pulsed currents provide non-plating periods forsupplying no plating current in the overall plating process, the metalion concentration in the plating solution within the through-hole isrecovered in the non-plating periods, for thereby preventing defectssuch as voids or the like from being formed in the plated film. As thethrough-hole is gradually filled with the plated film in the platingprocess, the substantive aspect ratio of the through-hole changes. Whenthe substantive aspect ratio of the through-hole changes, it is possibleto efficiently fill the plated film into the through-hole in a manner tomatch the changing substantive aspect ratio of the through-hole byincreasing the cathode current density on average in the platingprocess. Consequently, the period of time required to plate thesubstrate can be further shortened.

It is generally known in the art to increase the plating current densitystepwise as the plating process progresses. However, it is difficult toinhibit the generation of monovalent copper over a full range of platingcurrent densities from a low plating current density to a high platingcurrent density. According to this example, since the cathode currentdensity has a constant peak value to inhibit the generation ofmonovalent copper, the plating solution can be prevented from beingdegraded.

FIG. 26 shows the relationship between the cathode current density andtime for yet another example of a plating current that is suppliedbetween a surface of the substrate W and the insoluble anode 52 disposedin facing relation to the surface of the substrate W. The example shownin FIG. 26 is different from the example shown in FIG. 25 in that thereverse electrolyzing process B₁ shown in FIG. 23 is carried out byapplying two pulses each at the pulse pitch P₄ in the range from 0.1 to10 ms, for example, preferably from 0.5 to 1.0 ms, instead of thereverse electrolyzing process B shown in FIG. 25 with the single pulseat the pulse pitch P₃ in the range from 0.1 to 10 ms, preferably from0.5 to 1 ms, for example.

FIG. 27 shows the relationship between the cathode current density andtime for yet still another example of a plating current that is suppliedbetween a surface of the substrate W and the insoluble anode 52 disposedin facing relation to the surface of the substrate W. The example shownin FIG. 27 is different from the example shown in FIG. 25 in that thefirst, second and third plating processes A₁, A₂, A₃ have respectiveprocessing times which are equal to each other, the on/off pulsedcurrent in the first plating process A₁ has a pulse pitch P₅ shorterthan the pulse pitch P₆ of the on/off pulsed current in the secondplating process A₂ (P₅<P₆), the pulse pitch P_(o) of the on/off pulsedcurrent in the second plating process A₂ is shorter than the pulse pitchP₇ of the on/off pulsed current in the third plating process A₃ (P₆<P₇),the downtime pitch P₈ of the on/off pulsed current in the first platingprocess A₁ is longer than the downtime pitch P₉ of the on/off pulsedcurrent in the second plating process A₂ (P₈>P₉), and the downtime pitchP₉ of the on/off pulsed current in the second plating process A₂ islonger than the downtime pitch P₁₀ of the on/off pulsed current in thethird plating process A₃ (P₉>P₁₀). Therefore, the cathode currentdensity on average increases stepwise.

FIG. 28 shows the relationship between the cathode current density andtime for a further example of a plating current that is supplied betweena surface of the substrate W and the insoluble anode 52 disposed infacing relation to the surface of the substrate W. The example shown inFIG. 28 is different from the example shown in FIG. 25 in that it uses acomposite pulsed power source for supplying a first plating current witha positive cathode current density D₁ ranging from 1 to 3 ASD, forexample, and a second plating current with a positive cathode currentdensity D₄ ranging from 0.1 to 0.5 ASD, for example, instead of thepower source for supplying the on/off pulsed current by repeating thesupply and non-supply of a plating current which flows in the forwarddirection, i.e., the plating direction, and has a positive cathodecurrent density D₁ in the range from 1 to 3 ASD, for example.

Since the composite pulsed power source is used to continuously supply aweak current in the range from 0.1 to 0.5 ASD, for example, rather thanstopping to supply the plating current, the plated film is continuouslygrown in the plating process. Therefore, the plated film is preventedfrom being dissolved into the plating solution in the plating process.

FIG. 29 shows the relationship between the cathode current density andtime for a still further example of a plating current that is suppliedbetween a surface of the substrate W and the insoluble anode 52 disposedin facing relation to the surface of the substrate W. The example shownin FIG. 29 is different from the example shown in FIG. 25 in that a PRpulsed current is supplied by repeating normal electrolyzing cycles witha positive cathode current density D₁ in the range from 1 to 3 ASD, forexample, and reverse electrolyzing cycles with a negative cathodecurrent density D₂ in the range from −0.05 to −4 ASD, for example,rather than the on/off pulsed current supplied by repeating the supplyand non-supply of a plating current with a positive cathode currentdensity in the range from 1 to 3 ASD, for example.

FIG. 30 shows the relationship between the cathode current density andtime for a yet further example of a plating current that is suppliedbetween a surface of the substrate W and the insoluble anode 52 disposedin facing relation to the surface of the substrate W. The example shownin FIG. 30 is different from the example shown in FIG. 25 in that itcarries out first, second and third plating processes A₁, A₂, A₃successively, by supplying a DC plating current with a positive cathodecurrent density D₁ in the range from 1 to 3 ASD, for example, the first,second and third plating processes A₁, A₂, A₃ having respectiveprocessing times that are progressively longer in this order (A₁<A₂<A₃).

Depending on the aspect ratio of a through-hole, the structure of aplating underlayer, the nature of the plating solution, etc., there maybe no need to provide a quiescent period between reverse electrolyzingprocesses. If no quiescent period is required, then a plating currentmay be supplied between the surface of the substrate W and the insolubleanode 52 to achieve the relationship between the cathode current densityand time shown in FIG. 30 for thereby shortening the time required toperform the plating process to efficiently fill the plated film into thethrough-hole.

FIG. 31 shows the relationship between the cathode current density andtime for a yet still further example of a plating current that issupplied between a surface of the substrate W and the insoluble anode 52disposed in facing relation to the surface of the substrate W. Theexample shown in FIG. 31 is different from the example shown in FIG. 20in that when the plated film 106 is embedded to a predeterminedthickness in the recesses 108 in the through-hole 100 a, as shown inFIG. 1D, so that the danger of a pinch-off is reduced, for example, thereverse electrolyzing process B is followed by a plating process A₄which is carried out by supplying a DC plating current with a positivecathode current density D₁ in the range from 1 to 3 ASD, for example. Atthe stage wherein the danger of a pinch-off is reduced, the embedding ofthe plated film in the through-hole 100 a in the substrate W isessentially completed, as shown in FIG. 1D, and dimples left on thesurface of the substrate are to be finally filled up. At this time, itis not necessary to supply a DC plating current to equalize the cathodecurrent density with a previous pulse peak current density, but a DCplating current may be supplied to make the cathode current densityhigher than a previous pulse peak current density, thereby shorteningthe time required to perform the plating process.

FIG. 32 shows the relationship between the cathode current density andtime for another example of a plating current that is supplied between asurface of the substrate W and the insoluble anode 52 disposed in facingrelation to the surface of the substrate W. The example shown in FIG. 32is different from the example shown in FIG. 27 in that the third platingprocess A₃ is performed by supplying a DC plating current with apositive cathode current density D₁ in the range from 1 to 3 ASD, forexample, thereby shortening the time required to perform the platingprocess.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. An electroplating method comprising: immersing a substrate with athrough-hole defined therein in a plating solution in a plating tank;disposing a pair of anodes in the plating solution in the plating tankin facing relation to face and reverse sides, respectively, of thesubstrate in the plating solution; performing a plurality of platingprocesses, each for a predetermined period, on the face and reversesides of the substrate by supplying pulsed currents respectively betweenthe face side of the substrate and one of the anodes which faces theface side of the substrate, and between the reverse side of thesubstrate and the other of the anodes which faces the reverse side ofthe substrate; and performing a reverse electrolyzing process on theface and reverse sides of the substrate between adjacent ones of theplating processes by supplying currents in an opposite direction to thepulsed currents in the plating processes respectively between the faceside of the substrate and one of the anodes which faces the face side ofthe substrate, and between the reverse side of the substrate and theother of the anodes which faces the reverse side of the substrate.
 2. Anelectroplating method according to claim 1, wherein each of the pulsedcurrents comprises a PR pulsed current represented by an alternaterepetition of a current flowing in a forward direction and a currentflowing in a reverse direction.
 3. An electroplating method according toclaim 1, wherein each of the pulsed currents comprises an on/off pulsedcurrent represented by an alternate repetition of the supply andnon-supply of a plating current which flows in a forward direction. 4.An electroplating method according to claim 1, wherein each of thepulsed currents comprises a composite pulsed current represented by acombination of two pulsed currents having different current values. 5.An electroplating method according to claim 1, wherein the platingprocesses together with the reverse electrolyzing process are performedto gradually increase an average current density as the substrate isprogressively plated.
 6. An electroplating method according to claim 1,wherein the reverse electrolyzing process is performed a plurality oftimes before and after a normal electrolyzing cycle in which a pulsedcurrent is supplied in the forward direction.